1. Field of the Invention
This invention relates to electron beam sources and, more particularly, to photocathodes for the generation of single or multiple electron beams.
2. Prior Art
High resolution electron beam sources are used in systems such as scanning electron microscopes, defect detection instruments, VLSI testing equipment, and electron beam (e-beam) lithography. In general, e-beam systems include an electron beam source and electron optics. The electrons are accelerated from the source and focused to define an image at a target. These systems typically utilize a physically small electron source having a high brightness.
Improvements in optical lithography techniques in recent years have enabled a considerable decrease in the linewidths of circuit elements in integrated circuits. Optical methods, however, will soon reach their resolution limits. Production of smaller line-width circuit elements (i.e., those less than about 0.1 .mu.m) will require new techniques such as X-ray or e-beam lithography.
In e-beam lithography, a controllable source of electrons is desired. A photocathode used to produce an array of patterned e-beams is shown in FIG. 1. U.S. Pat. No. 5,684,360 to Baum et al., "Electron Sources Utilizing Negative Electron Affinity Photocathodes with Ultra-Small Emission Areas," herein incorporated by reference in its entirety, describes a patterned photocathode system of this type.
FIG. 1 shows a photocathode array 100 with three photocathodes 110 comprising a transparent substrate 101 and a photoemission layer 102. The photocathode is back-illuminated with light beams 103 which are focused on photoemission layer 102 at irradiation region 105. As a result of the back-illumination onto photoemission layer 102, electron beams 104 are generated at an emission region 108 opposite each irradiation region 105. Other systems have been designed where the photoemitter is front-illuminated, i.e. the light beam is incident on the same side of the photoemitter from which the electron beam is emitted.
Often, light beams 103 or electron beams 104 are masked. In FIG. 1, light beams 103 are masked using mask 106 which allows light onto irradiation spots 108 but prevents light from being incident on other areas of photoemission layer 102. FIG. 1 also shows mask 107 which allows electrons to exit photoemission layer 102 only at certain surface spots corresponding to emission regions 105. A photocathode may also have a mask between transparent substrate 101 and photoemission layer 102 to block light beam 103 so that it is only incident at irradiation spots 105. In general, photocathode 110 may include no masking layers or may have one or more masking layers.
Each irradiation region 105 may be a single circular spot representing a pixel of a larger shape, the larger shape being formed by the conglomerate of a large number of photocathodes 110 in photocathode array 100. In that case, irradiation region 105 may be as small as is possible given the wavelength of the light beam incident on photocathode 100. Typically, a grouping of pixel irradiation regions has dimensions of 100-200 .mu.m. Each pixel can have dimensions (i.e. diameter) as low as 0.1 .mu.m. Alternatively, irradiation spot 105 and emission region 108 can be a larger shape. In either case, the image formed by emission region 108 will be transferred to e-beam 104 so long as the entirety of irradiation region 105 is illuminated by light beam 103.
Photoemission layer 102 is made from any material that emits electrons when irradiated with light. These materials include metallic films (gold, aluminum, etc.) and, in the case of negative affinity (NEA) photocathodes, semiconductor materials (especially III-V compounds such as gallium arsenide). Photoemission layers in negative electron affinity photocathodes are discussed in Baum (U.S. Pat. No. 5,684,360).
When irradiated with photons having energy greater than the work function of the material, photoemission layer 102 emits electrons. Typically, photoemission layer 102 is grounded so that electrons are replenished. Photoemission layer 102 may also be shaped at emission region 108 in order to provide better irradiation control of the beam of electrons emitted from emission region 108. Further control of the e-beam is provided in an evacuated column as shown in FIG. 2.
Light beams 103 usually originate at a laser but may also originate at a lamp such as a UV lamp. The laser or lamp output is typically split into several beams in order to illuminate each of focal points 105. A set of parallel light beams 103 can be created using a single laser and a beam splitter. The parallel light beams may also originate at a single UV source. Alternatively, the entire photoemission array 100 may be illuminated if the light source has sufficient intensity.
Photons in light beam 103 have an energy of at least the work function of photoemission layer 102. The intensity of light beam 103 relates to the number of electrons generated at focal point 105 and is therefore related to the number of electrons emitted from emission region 108. Photoemission layer 102 is thin enough and the energy of the photons in light beam 103 is great enough that a significant number of electrons generated at irradiation region 103 migrate and are ultimately emitted from emission layer 108.
Transparent substrate 101 is transparent to the light beam and structurally sound enough to support the photocathode device within an electron beam column which may be a conventional column or a microcolumn. Transparent substrate 101 may also be shaped at the surface where light beams 103 are incident in order to provide focusing lenses for light beams 103. Typically, transparent substrate 101 is a glass although other substrate materials such as sapphire or fused silica are also used.
If mask 106 is present either on the surface of transparent substrate 101 or deposited between transparent substrate 101 and photoemission layer 102, it is opaque to light beam 103. If mask 107 is present, it absorbs electrons thereby preventing their release from emission region 108. Mask 107 may further provide an electrical ground for photoemission layer 102 provided that mask 107 is conducting.
Photocathode 100 may be incorporated within a conventional electron beam column or a microcolumn. Information relating to the workings of a microcolumn, in general, is given in the following articles and patents: "Experimental Evaluation of a 20.times.20 mm Footprint Microcolumn," by E. Kratschmer et al., Journal of Vacuum Science Technology Bulletin 14(6), pp. 3792-96, November/December 1996; "Electron Beam Technology--SEM to Microcolumn", by T. H. P. Chang et al., Microelectronic Engineering 32, pp. 113-130, 1996; "Electron Beam Microcolumn Technology And Applications", by T. H. P. Chang et al., Electron-Beam Sources and Charged-Particle Optics, SPIE Vol. 2522, pp. 4-12, 1995; "Lens and Deflector Design for Microcolumns", by M. G. R. Thomson and T. H. P. Chang, Journal of Vacuum Science Technology Bulletin 13(6), pp. 2445-49, November/December 1995; "Miniature Schottky Electron Source", by H. S. Kim et al., Journal of Vacuum Science Technology Bulletin 13(6), pp. 2468-72, November/December 1995; U.S. Pat. No. 5,122,663 to Chang et al.; and U.S. Pat. No. 5,155,412 to Chang et al., all of which are incorporated herein by reference.
FIG. 2 shows a typical electron beam column 200 using photocathode array 100 as an electron source. Column 200 is enclosed within an evacuated column chamber (not shown). Photocathode array 100 may be completely closed within the evacuated column chamber or transparent substrate 101 may form a window to the vacuum chamber through which light beams 103 gain access from outside the vacuum chamber. Electron beams 104 are emitted from emission region 108 into the evacuated column chamber and carry an image of emission region 108. Electron beam 104 may be further shaped by other components of column 200.
Electron beams 104 are accelerated between photocathode array 100 and anode 201 by a voltage supplied between anode 201 and photoemission layer 102. The voltage between photocathode array 100 and anode 201, created by power supply 208 (housed outside of the vacuum chamber), is typically a few kilovolts to a few tens of kilovolts. The electron beam then passes through electron lens 204 that focuses the electron beam onto limiting aperture 202. Limiting aperture 202 blocks those components of the electron beams that have a larger emission solid angle than desired. Electron lens 205 refocuses the electron beam. Electronic lenses 204 and 205 focus and demagnify the image carried by the electron beam onto target 207. Deflector 203 causes the electron beam to laterally shift, allowing control over the location of the image carried by the electron beam on a target 207.
In 0.1 .mu.m lithography systems, the size of a circular pixel incident on target 207 is on the order of 0.05 .mu.m. Therefore, the image of emission area 108 needs to be reduced by roughly a factor of 2 to 10, depending on the size of emission region 108. Target 207 may be a semiconductor wafer or a mask blank.
Conventional variable shaped electron beam lithography columns shape the electron beam by deflecting the electron beam across one or more shaping apertures. The resulting image in the shaped electron beam is then transferred to target 207 with a large total linear column demagnification. The requirement of large total linear demagnification (supplied by electron lenses 204 and 205) results in large column lengths, increasing electron-electron interactions that ultimately limit the electron current density of the column. The low electron current density results in a low throughput when the column is used in lithography.
Another major drawback in using known e-beam systems include the inability to modulate the electron beam without modulating the light source itself, usually a laser. Modulating a laser typically involves a large amount of control circuitry, requiring a large amount of space, and can be slow. In addition, in a patterned array of photocathodes, modulation of individual photocathodes in the array is extremely difficult. Finally, better resolution is required of lithography systems in order to meet future demands of semiconductor materials processing.